Temperature maps of surface water in the North Atlantic for 18,000 B.P. have been reconstructed for the four seasons. Temperatures were estimated by transfer-function analysis of foraminiferal assemblages, and geometric patterns of surface waters were derived from water-mass-related assemblages of Coccolithophorida and Foraminifera.

At 18,000 B.P. the Arctic Polar Front, which was characterized by a steep thermal gradient parallel and centered on lat 42°N, marked the fundamental dividing line for all climatic regimens between a northern dynamic zone and a southern area of relative stability.

North of lat 42°N the glacial Atlantic was polar in character with wide areas of seasonal pack ice. The Norwegian-Greenland and Labrador Seas had year-round ice cover. The polar sea was dominated by a counterclockwise gyre.

Subpolar and transitional surface water masses were squeezed into a narrow band between the polar front and the subtropical gyre, whose geometry differed only slightly from today. Increased upwelling occurred off the west coast of Africa, while the equatorial divergence-Benguela flow increased during Southern Hemisphere winter at 18,000 B.P.

The greatest temperature differences between today and 18,000 B.P. are found in a latitudinal band from 42°N to 60°N, with differences in some areas exceeding 10°C. North and south of this maximum anomaly band, temperature differences are smaller. The upwelling region off Africa shows a 6°C anomaly. The subtropical gyre shows no statistically significant anomaly.